Redox titration of Betadine®Material

(1)

Health

Lab session Redox titration of Betadine®

Material Teacher’s desk

S

0

Betadine® yellow C

0

= [ I

2

]

S

2

Sodium thiosulphate solution C

2

= 1,00 .10

^-2

mol.L

^-1

 50cc beaker (S

0

)

 10cc volumetric pipette + Bulb filler

 100cc volumetric flask (S

1

)

 2 x 100cc beakers

 Red pencils for glassware

 Sponge + Towel + Paper roll

For each group

 Iodine solution ≈ 1,00 .10

^-2

mol.L

^-1

 2 x 50cc beakers

 25cc graduated burette

 3cc disposable polyethylene pipette

 150cc Erlenmeyer

 20cc graduated cylinder

 Magnetic stirrer + magnetic rod

 Squeezing bottle of distilled water Theoretical goals

 Understand the principle of an oxidation-reduction titration;

 Identify an oxidizing agent and a reducing agent;

 Find the redox reaction to realize an iodine solution titration. Identify the 2 redox pairs;

 Define equivalence of a titration and thereafter deduct the amount of matter of the titrated reagent.

Experimental goals

 Dilution of the mother solution;

 Glassware choice and use;

 Spot the equivalence during the follow-up of a colorimetric titration;

 Check the iodine molar concentration in the yellow Betadine® solution.

I. Principle of a colorimetric titration

The orange-brown coloration of Bétadine® (solution S

0

) is due to dissolved iodine (I

2

).

 In a beaker, pour a small amount (≈ 2 cc) of iodine solution (I

2

) ;

 While stirring, add drop by drop a sodium thiosulphate solution to finally obtain the yellow colour fades away.

This chemical reaction produces colourless iodine ions (I

^-

) and colourless tetrathionate ions (S

4

O

62-

).

a) Give the two involved redox pairs and write their associate half-equations.

b) To sum-up, write the redox reaction necessary to realize the iodine solution titration. Frame the oxidizing agent.

c) Recall the characteristics of a titration reaction?

d) Recall the definition of equivalence.

e) Before equivalence:

- What is colour of the solution inside the beaker?

- Which reagent is in excess?

- Which reagent is limiting?

After equivalence:

- What is colour of the solution inside the beaker?

- Which reagent is in excess?

- Which reagent is limiting?

f) How will you spot the equivalence during the follow-up process?

1 tempfile_12114.doc

(2)

Health II. Protocol

1. Dilution of the mother solution

 Before the titration, the betadine solution has to be diluted with a dilution factor of 5 in order to prepare a daughter solution S

1

for the class.

What is the required volume V

0

of mother solution to obtain a final volume V

1

= 100,0 mL? Justify.

 Protocol

 Preparation of the titrated solution

Using a volumetric pipette, take a volume V

1

= 10,0 mL of the daughter solution (S

1

) and pour it in an erlenmeyer flask ; Using a graduated cylinder, add around 20 mL of distilled water to it, and … the magnetic rod.

 Preparation of the titrating solution

Rinse the burette with distilled water, then with potassium thiosulphate solution, fill it up to the «zero » mark, and check no bubbles are left in the tap.

 Titration

Place the erlenmeyer flask under the burette, add a white sheet of paper right under it to better check the colour change, set the magnetic stirrer and start to pour the titrating solution.

Just before equivalence, the orange colour fades. Add some starch solution. Take note of the colour change.

Drop by drop, keep pouring the titrating solution to finally obtain the blue colour fades away.

Take note of the added volume at equivalence: V

E

= Eventually realize a second titration to double check it.

2. Exploitation

a) For what reason did we add the starch solution?

b) Does the added water in the Erlenmeyer alter the volume at equivalence?

c) Fill up the extend table of the redox reaction at equivalence. (Just letters, no value!)

Initial state x = 0 Medium state x ≠ 0 Equivalence x

max

=

d) At equivalence, the change of limiting reagent implies: n(I

2

, final) = 0 n(S

2

O

32-

, final) = 0 Find the value of the extend x

max

at equivalence.

e) Give the relation between the amounts of matter at equivalence?

f) Deduct the molar concentration C

1

according to V

1

, C

2

and V

E

g) Calculate its numerical value .

h) Deduct the experimental concentration C

0

(exp) of Betadine®.

i) Calculate the theoretical value C

0

(th) of its concentration.

Betadine®’s label stands:

povidone-iodine: 10%

(1% available iodine) Data : µ

Betadine

≈ µ

water

= 1000 g.L

^-1

M(I

2

) = 253,8 g.mol

^-1

Conclude !

2 tempfile_12114.doc

(3)